1. Field of the Invention
The present invention relates to a device for vacuum sealing containers, and in particular to a device for sensing the presence of a fluid pumped out of a container, and converting the sensor output to a signal for indicating the formation of a vacuum within the container.
2. Description of the Related Art
Various apparatus and methods are known for the purpose of vacuum sealing containers to protect perishables provided therein, such as foodstuffs and other products, against oxidation. One type of vacuum sealing system, primarily used for commercial packaging purposes, includes a vacuum chamber in which the entire packaged product is placed, along with heat sealers for sealing the package once a vacuum has been substantially established within the interior of the package.
Another type of conventional vacuum sealing system is manufactured to be more compact and economical for home use. One such system is disclosed in applicant's U.S. Pat. No. 4,941,310, previously incorporated by reference, which in one embodiment discloses a vacuum chamber including an opening defined by a stationary support member and a moveable hood. An open end of a container such as a bag to be sealed is received within the vacuum chamber between the support member and the moveable hood, such that when the hood is moved to a closed position, a sealed environment including the vacuum chamber and the interior of the bag is established. A preferred type of bag for use with such a system is disclosed in applicant's U.S. Pat. No. 4,756,422, entitled, "PLASTIC BAG FOR VACUUM SEALING", which bag is provided with a series of air channels on interior surfaces of the bag. The air channels allow fluid flow from the bag into the vacuum chamber, thereby allowing evacuation of the bag even though the open end of the bag is firmly held between the support member and moveable hood.
After the moveable hood is located in the closed position with the open end of the bag located within the vacuum chamber, a pump within the device evacuates the fluid from within the bag. Once a vacuum is substantially established within the bag, a heat source seals the opening of the bag thereby vacuum sealing the perishable goods within the bag.
Systems for vacuum packaging perishable items such as those described above conventionally employ pressure sensors for determining when a sufficient vacuum is established within the vacuum chamber and vacuum-seal bag. Such pressure sensors conventionally operate by comparing the interior chamber/container pressure to a reference pressure, which is generally ambient pressure. A control mechanism shuts down the evacuation pump when a pressure differential between the chamber/container interior and reference pressures reaches a predetermined value, thereby indicating a substantial vacuum within the chamber and container. However, a shortcoming with conventional pressure sensors used in vacuum packaging devices is that the reference pressure may change significantly with a change in temperature and/or elevation. For example, if a vacuum packaging device including a conventional pressure sensor is used in a low elevation/high pressure location, the predetermined pressure differential between the chamber/container interior and reference pressure may be reached prematurely, and the pump may be shut down prior to complete evacuation of the fluid from within the container to be vacuum sealed. Conversely, if a vacuum packaging device including a conventional pressure sensor is used at a high elevation/low pressure location, the predetermined pressure differential may never be reached, and consequently the evacuation pump will continue to operate even though a vacuum has been substantially established within the vacuum-seal container.